EMI filters to suppress noise generated from electronic devices are more widely used in various applications as semiconductor integrated circuits are increased in integration and provided with high performance. In addition to suppressing noise, EMI filters enhanced with ESD protection have been recently developed for the ESD protection to prevent elements of the EMI filters from destruction due to external high-voltage surges. Various types of EMI filters are available for ordinary signal lines, high-speed signal lines and power lines: an RC type composed of resistor components and capacitors, an LC type composed of inductors and capacitors, an inductor type, a capacitor type, etc. Since Zener diodes have a highly protective capability for external high-voltage surges, such Zener diodes are applied for ESD protection. Capacitive components of ESD protective diodes are used for capacitors and filters composed of such capacitors and resistor components suppress noise at ordinary time when no ESD occurs, so that low-pass filters and the like have been developed (as disclosed in Vishay Inc., Data Sheet “CTF701-HS3 (2 Line EMI-Filter with ESD-Protection).
In the case of the RC type EMI filters or the like, however, where a resistor component that exhibits a small impedance in a low frequency band is used for the purpose of reduction of an insertion loss, a large current flows through the resistor component and ESD protective diodes at the occurrence of external high-voltage surges, so that required ESD endurance cannot be sustained. On the other hand, where a resistor component that exhibits a large impedance in a low frequency band is used, required ESD endurance can be sustained but a required insertion loss cannot be maintained.